Parts to fix Hubble may not be ready Experts seeking cause of problem

CAPE CANAVERAL, Fla. -- With its long-awaited mission to fix the Hubble Space Telescope just 10 weeks away, NASA is not sure the replacement parts are ready to fly.

Two dozen optics experts gathered yesterday at Goddard Space Flight Center in Greenbelt, Md., to determine whether discrepancies in previous tests were the fault of the test equipment or the two replacement instruments, which have already been shipped to Kennedy Space Center for launch aboard a space shuttle Dec. 2.

Mr. McCarthy said NASA would not launch the shuttle Endeavour with the replacement parts aboard until the test data was thoroughly understood. "It will be resolved," he said yesterday.

The $1.5 billion Hubble telescope was placed in orbit by a NASA shuttle in 1990 with a flaw in its primary mirror. It was later determined that the mistake hadn't been discovered while the telescope was still on the ground because of faulty test equipment.

As a result, the telescope can't focus properly. The replacement parts built for the repair mission are designed to correct the flaw and eliminate the telescope's blurry vision.

NASA is worried that mirrors in the two replacement instruments -- the large Corrective Optics Space Telescope Axial Replacement and the smaller Wide-Field/Planetary Camera -- won't focus light from the flawed primary mirror to the same point, or focal plane.

Both parts are to be installed by astronauts during an 11- to 13-day flight that could include as many as five to seven spacewalks.

The wide-field camera would replace one already on Hubble, while the other instrument, known as COSTAR, would slide in between the main mirror and other instruments to intercept and correct the distorted light.

The problem, Mr. McCarthy said, is that tests conducted before the instruments left Goddard for the space center could mean that the focal lengths for the wide-field camera and COSTAR -- the distance from the mirrors to the point at which the light comes into focus -- are not close enough to each other to be adjusted by remote control once repairs are made.

On the other hand, the data might be the result of a flaw in the test equipment or in the way the tests were conducted, Mr. McCarthy said -- the more likely explanation, he added.

In either case, he said, agency managers want to know for sure before the instruments are placed aboard Endeavour and launched into a 370-mile-high orbit, beyond the reach of Goddard engineers.

NASA managers may decide as early as today or this weekend whether one of the instruments must go back to Goddard for another set of tests, Mr. McCarthy said.